An input stage circuit is a basic building block for analog or mixed signal circuit. A first stage of an analog or mixed signal circuit may typically include a rail-to-rail (R-R) input stage circuit operating over a rail-to-rail input common mode voltage range. An example of such an analog or mixed signal circuit is an operational amplifier, a comparator or an analog-to-digital converter. An R-R input stage circuit is designed to handle a wide input common-mode voltage range. Typically, the R-R input stage circuit may handle an input common mode voltage ranging from a negative supply (rail) or ground to a positive supply (rail), hence the name rail-to-rail.
Analog or mixed signal circuits used in low-voltage CMOS integrated-circuit manufacturing processes normally require a differential input stage circuit that can operate even when its input common mode voltage is near the supply rails.
Conventional R-R input stage circuits use two complimentary differential transistor pairs. The first differential pair (N_diff_pair) includes a pair of matched NMOS transistors and the second differential pair (P_diff_pair) includes a pair of matched PMOS transistors. FIG. 1 shows a typical N_diff_pair. The input common mode of differential pairs is limited. For N_diff_pair 10, the input common mode is limited to the voltage range from VSS+Vth+2*VDsat to VDD. A saturation voltage for M0, M1 and M2 is defined as VDsat. A threshold of MOS transistors M1 and M2 is defined as Vth. VSS and VDD being the negative and the positive supply, respectively. FIG. 2 shows a typical P_diff_pair. For P_diff_pair 20, the input common mode is limited to the voltage range from VSS to VDD−Vth−2*VDsat. Thus if VDD>VSS+2*(Vth+2VDsat), the combination of a P_diff_pair and an N_diff_pair can be used to support the entire common mode range from the negative to the positive supply.
FIG. 3 is an example of an R-R input stage circuit including complementary differential pairs of transistors. R-R input stage circuit 30 includes first stage circuit 300 and second stage circuit 330. First stage circuit 300 includes first differential pair 310 and second differential pair 320. The first differential pair is a P_diff_pair and the second differential pair is an N_diff_pair, similar to the P_diff_pair and the N_diff_pair of FIG. 2 and FIG. 1, respectively. R-R input stage circuit 30 shown in FIG. 3 is also known as a two stage folded cascode amplifier.
Second stage circuit 330 combines the current flowing through P_diff_pair 310 and N_diff_pair 320. These currents inherently flow in opposite directions. Therefore, a second stage is required to fold and sum the currents flowing through P_diff_pair 310 and N_diff_pair 320.
A drawback of R-R input stage circuit 30, which uses complementary differential pairs, is the requirement of a second stage circuit (which in the example is second stage circuit 330). The second stage circuit limits the bandwidth and operational speed of the R-R input stage circuit. As a result, the bandwidth and speed of any circuit embodying the R-R input stage is also limited.
Furthermore, it is a design challenge to optimize the design of R-R input stage circuit 30 for noise, power consumption and speed.
More recently, R-R input stage circuits are developed based on non-complementary differential pairs. They include two sets of same type of differential pairs to overcome the shortcomings of the R-R input stage circuits that use complementary differential pairs.
While these known input stage circuits offer a satisfactory performance and the ability to operate over a wider common mode range reaching from the negative power supply potential up to the positive power supply potential, there remains the desire to further improve the performance of an input stage circuit. Improvements are particularly directed to overcome performance issues that appear as the input common-mode voltages are near the supply rails. As the input common mode approaches the supply rails, the internal bias points change significantly. This, in turn, can lead to degradation in performance of the R-R input stage circuit and consequently of the entire analog or mixed signal circuit that follows the R-R input stage circuit.
Biasing circuitry to improve rail-to-rail operation of non-complementary differential pair input stage circuits is necessary for very low supply voltage operation.